Light source wavelength correction

ABSTRACT

A wavelength correction function provides corrected reflectance values from actual reflectance values taken in a reflectance-base instrument. The correction is provided as a function of measured reflectance values and a predefined set of high resolution reflectance values established for the reflectance-based instrument implementing the wavelength correction function.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. §119(e)from co-pending, commonly owned U.S. provisional patent application Ser.No. 60/475,288, entitled DIAGNOSTIC INSTRUMENT, filed Jun. 3, 2003.

This application claims the benefit of priority under 35 U.S.C. §120from co-pending, commonly owned U.S. non-provisional patent applicationSer. No. 10/821,441, entitled TRAY ASSEMBLY FOR OPTICAL INSPECTIONAPPARATUS, filed Apr. 9, 2004.

FIELD OF THE INVENTION

The inventive concepts relate to reflectance-based systems and methods.More particularly, the present invention relates to systems and methodsfor wavelength correction within such systems and methods.

BACKGROUND

Reflectance-based instruments have long been in use in a variety ofapplications. One type of reflectance-based system is referred to as a“reflectometer”, used to perform tests in certain medical and laboratoryapplications. In a typical form, a reflectometer includes one or morelight sources configured to generate one or more light signals at givenwavelengths. An object under test receives the signal and reflects aportion thereof—referred to as “reflectance”. Reflectance is typicallyconsidered to be unit-less because it is defined as the ratio of thelight actually leaving a sample to the amount that would leave if nonewere absorbed. In recent years, the National Institute of Standards andTechnology (NIST) has defined reflectance in terms of this kind ofmathematical model, rather than provide a physical reflectance standard.The perfect diffuse surface scatters light according to Lambert's law,which states that the intensity of light scattered from a point on areflecting surface follows a cosine relationship with the polar angle ofthe scattered light, independent of the direction of the incident light.One or more detectors or sensors are oriented to receive the reflectedsignals. A processor analyzes the characteristics of the receivedreflected signals and produces a test result.

Such reflectometers are sometimes used for performing tests on a reagenttest strip. In such a case, the test pads on the test strip may beincrementally tested to determine the presence of analytes in a liquidtest sample absorbed into the test pads. Such systems may be used forperforming urinalysis tests, as one example. That is, the level orpresence of an analyte in a liquid test sample can be determined bycausing a given test pad to absorb some of the liquid test sample,(e.g., a sample of urine) and then by reading associated reflectancevalues for the test pad with a reflectometer. Based on the spectralreflectance characteristics of the signal reflected by the test pad, thereflectometer determines the presence or level of the analyte in a giventest pad. As an example, a test pad changes color to indicate the levelor presence of the analyte in response to absorption of urine from aurine sample. The characteristics of a reflected signal are a functionof the make-up and color of the test pad and the wavelength of the lightsource. Consequently, a change in color of a test pad causes acorresponding change in the characteristics of the reflected signal.

Test strips are typically produced according to industry acceptedformats. In the case of urinalysis reflectometers, test strips can comein formats having different lengths, such as, for example, 3.25 inchlengths or 4.25 inch lengths. Within each format, a test strip is defmedaccording to its configuration, i.e., the number, types and order oftest pads included on the test strip. Generally, each test stripconfiguration is uniquely identified., Implicit in a test stripidentification and/or confirmation, therefore, is the test strip formatand the test pad configuration. As will be appreciated by those skilledin the art, such test pads may include, for example, pH, ketone,nitrite, and glucose test pads. In order for the reflectometer toproduce valid results, the test strip must be identified by format andconfiguration, so that the reflectometer has a proper context toevaluate the received reflected signals, or reflectance values derivedtherefrom. That is, a reflectometer needs to know that a receivedreflected signal is produced by, for example, a glucose test pad or aketone test pad.

Reagent cassettes can also tested using a reflectometer, in a mannervery similar to that used for the test strip. Such reagent cassettesinclude a test region that provides visual indications of test results,similar the test pads of the test strips. The test region can produce aseries of lines that embody the test results.

There is a variety of known ways that the test strip is identified to orby the reflectometer. In some reflectometers, an operator enters datainto the reflectometer that indicates the identification of the teststrip from a look up table, or chooses the identification from a set ofpredefined options. The same can be done for reagent cassettes. Thereflectometer is then ready to process the test strip or cassette.

In typical reflectance-based instruments, such as reflectometers, lightemitting diodes (LEDs) serve as sources of light, the reflections ofwhich are then detected and evaluated. Each LED is specified to have acenter wavelength, within some range. Depending on the application, therange can be relatively narrow, e.g., ±3 nanometers (nm). Wavelengthsoutside of this range can result in instrument errors or incorrectclinical results. Since there is no way to correct for LEDs havingcenter wavelengths outside the specified range, presently, instrumentmakers are reliant on the relatively expensive process of sortingthrough large volumes of LEDs to find those that are within thespecified range. Otherwise, reflectance-based instruments using lessprecise LEDs would be error prone.

SUMMARY OF THE INVENTION

In accordance with aspects of the present invention, provided is asystem and method for correcting one or more reflectance values when acenter wavelength of one or more light sources used to generatecorresponding source light signals is different from a specified centerwavelength for the one or more light sources. The present invention canbe implemented as part of, or in conjunction with, any reflectance basedsystem. And the light sources may be LEDs, or any other type of lightsource.

The system and method comprise defining, for each of the one or morelight sources, a reference spectral distribution {L*} that ischaracteristic for the one or more light sources and comprised ofreference light intensity values over a set of reference wavelengths.Also determined, for each of the one or more light sources, is aspectral distribution {L} comprising actual light intensity values overthe set of wavelengths. The actual reflectance R of a set of reflectedsignals is determined, e.g., through detection and measurement.

For a set of detectors used as part of the reflectance-based system, aset of detector sensitivity data {D} is also stored. And a set of highresolution reflectance values {r} is determined. The correction functionuses {L}, {L*}, {r} and {D} to determine a correction factor. Thecorrection factor is applied to the measured reflectance R to determineR*. Corrected reflectance R* is then used in the calculations andfunctions of the instrument that would typically use R.

Determining {r} may comprise measuring reflectance values R_(IR) in theinfrared range and determining r_(IR) as a constant representing anaverage of R_(IR) values, where each value in {r} equals the value of(R/R_(IR))·r_(IR) at a corresponding wavelength. Additionally, thevalues of {r} can be determined at discrete wavelength intervals, wherethe intervals are sufficiently narrow, e.g., 1 nanometer, to provideclose correlation to the actual wavelengths of the R values.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawing figures depict preferred embodiments by way of example, notby way of limitations. In the figures, like reference numerals refer tothe same or similar elements.

FIG. 1 is a perspective view of a reflectometer that may implementwavelength correction in accordance with the present invention.

FIG. 2 is a view of a carriage used with the spectrometer of FIG. 1,including a view of a insert used with the carriage for accommodatingvarious test strips.

FIG. 3A and FIG. 3B are diagrams depicting a prior art arrangement offunctional elements that can be used within the reflectometer of FIG. 1.

FIG. 4 is a graph of representative plots of LED spectral distributionsfor each of the six LEDs of the reflectometer of FIG. 1 and FIGS. 3A-B,and used in the wavelength correction of FIG. 5.

FIG. 5 is a block diagram depicting wavelength correction in accordancewith aspects of the present invention, within the context of thereflectometer of FIG. 1, FIG. 3A and FIG. 3B.

FIG. 6 is a representative plot of detector sensitivity for CCDs of thereflectometer of FIG. 1 and FIGS. 3A-B and used in the wavelengthcorrection of FIG. 5.

FIG. 7A and FIG. 7B are representative plots of reference highresolution reflectance values at various color block levels for Glucoseand pH, used in the wavelength correction of FIG. 5.

FIG. 8A and FIG. 8B are representative plots of wavelength correctionvalues at various color block levels for Glucose and pH, used in thewavelength correction of FIG. 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with various aspects of the present invention, awavelength correction function provides corrected reflectance values ina reflectance-base instrument. The correction is provided as a functionof measured reflectance values and a predefined set of high resolutionreflectance values established for the reflectance-based instrumentimplementing the wavelength correction function. Without wavelengthcorrections, reflectance-based instruments are intolerant of lightsources, e.g., light emitting diodes (LEDs), having center wavelengthsbeyond narrowly specified wavelength ranges. Reflectance-basedinstruments having light sources with less reliable center wavelengthsare relatively error prone.

Representative Reflectance-Based Instrument

FIG. 1 provides a perspective view of an embodiment of a reflectometer100, as one example of a reflectance-based instrument, that may includefunctionality that implements the wavelength correction of the presentinvention. As will be appreciated by those skilled in the art, thepresent invention could be implemented in other reflectometers orreflectance-based instruments, so is not restricted to embodimentsprovided herein. Reflectometer 100 provides an input and output devicein the form of a touch screen 120. An output port 140 may be provided asa means for printing a report (e.g., test or diagnostic report) to anoperator or user of reflectometer 100. As will also be appreciated bythose skilled in the art, other forms of input and output mechanisms maybe used. For example, reflectometer 100 may be configured to couple, bywired or wireless means, to a personal computer, handheld computer,network, monitor, printer, audio/visual system or the like. A housing110 houses the touch screen 120, as well as a variety of internalfunctional elements. An input port 130 is provided to facilitateinsertion of one or more test strips or reagent cassettes (collectively,“test product(s)”) via a carriage.

Referring to FIG. 2, a collection of test product insertion components200 for use with reflectometer 100 is shown. A carriage 240 isconfigured for insertion in input port 130 of the reflectometer 100,with a test product. Carriage 240 includes an insert region 210 withinwhich a test product insert 220 configured to hold a test product (e.g.,a reagent test strip 290 or cassette 250) may be placed. In thepreferred form, insert 220 includes a first side 214 configured to holdthe reagent test strip 290 within a slot 216. Representative test strip290 includes a plurality of test pads 292, the configuration of whichdepends on the particular test strip type. Test strip 290 is positionedwithin slot 216 after the insert 220 is loaded into carriage 240 withside 214 available for testing. Carriage 240 may be configured toaccommodate a test strip 290 of any of a variety of lengths, such astest strips of the 3.25″ and 4.25″ length formats, as examples.

A region of interest to be tested may include one or more of test pads292. In order for the test pads 292 to be tested, those pads must bedisposed to receive light from the LEDs and to reflect light fordetection by light detectors, as described with respect to FIG. 3A andFIG. 3B below. Accordingly, in the embodiment of FIG. 2, test strip 290is disposed within carriage 240 such that the test strip pads 292 arevisible to such components.

Insert 220 may optionally include a second side 212 configured to acceptreagent cassette 250. Such reagent cassettes are known in the art. Forinstance, reagent cassette 250 may be a disposable, single-use hCGimmunoassay cassette for performing a pregnancy test. The reagentcassette 250, as with the test strip 290, includes a region of interestthat may include a test area defined by a window 254 and also includeidentification markings, such as bar codes 256. The reagent test area isviewable and capable of being tested when the carriage is loaded intoreflectometer 100.

The reagent cassette 250 has an opening or well 252 into which a bodyfluid sample, such as urine, is deposited. The fluid sample propagatesto the test area defined by window 254. The reagent cassette test areacomprises a test line area, reference line area and control line area,as is known in the art. Test results can take the form of one or morelines displayed in these areas. With introduction of a fluid sample, thereagent cassette test area may change color, for example, at least onecolored stripe may appear in window 254.

As an example, the various components of FIG. 2 may take the form ofthose more fully described in co-owned and co-pending U.S. patentapplication Ser. No. 10/821,441, entitled TRAY ASSEMBLY FOR OPTICALINSPECTION APPARATUS, filed Apr. 9, 2004.

FIGS. 3A and 3B show two different views of a prior art embodiment ofvarious functional elements that may be used for performingreflectance-based testing of a test product within reflectometer 100. Atop view is shown in FIG. 3A and a side view is shown in FIG. 3B. As isshown in each of the figures, test signals are provided by transmitters302. In this form, transmitters 302 are LEDs, preferably six, as shownin FIG. 3A, each of which transmits a different signal having a uniquewavelength.

In this embodiment, the signals transmitted by the LEDs are:

-   -   1) LED 1: blue light at a center wavelength of about 470        nanometers (nm),    -   2) LED 2: green light at a center wavelength of about 525 nm,    -   3) LED 3: green light at a center wavelength of about 565 nm,    -   4) LED 4: red light at a center wavelength of about 625 nm,    -   5) LED 5: red light at a center wavelength of about 660 nm, and    -   6) LED 6: infrared (IR) radiation at a center wavelength of        about 845 nm.

Test signals from LEDs 302 are transmitted through a guide 304 in thedirection of arrow A. The test signals from guide 304 impinge on teststrip 290 at an angle of about 45°, in the illustrative embodiment. Inthis embodiment, test strip 290 is housed within carriage 240. Lightreflected from the test strip in the direction of arrow B passes throughaperture 342, after which it impinges on a planar or convex mirror 330(not shown in FIG. 3A), which redirects and focuses the reflectedsignals in the direction of arrow C. In this arrangement, due to theorientation of mirror 330, the path of the reflected signals takes abouta 90° turn after leaving the test strip 290. The reflected signalspropagating in the direction of arrow C pass through aperture 340 andconverge at aspheric lens 350. Aspheric lens 350 focuses the reflectedsignals and the focused reflected signals continue to propagate in thedirection of arrow C. The reflected signals impinge on detector 360. Aswill be appreciated by those skilled in the art, the shapes andarrangement of mirrors and lenses need not specifically conform to or belimited to those shown in the illustrative embodiment of FIGS. 3A and3B.

As previously mentioned, detector 360 receives the reflected signals,translates them into an image comprised of data representing reflectancevalues associated with the test pads 292, and tests results therefrom.In this embodiment, detector 360 is a charge coupled device (CCD)comprised of a matrix of 2048 pixels configured to receive the reflectedsignals. Data from the reflected signals are recorded pixel-by-pixel asthe reflectance values. Pixel data are grouped and associated withindividual pads on the test strip 290. As a result, reflectance valuesfor each pad of the test strip 290 are stored.

Center Wavelength Correction

Each LED used in a reflectometer is rated at or identified as having apredefined spectral distribution that includes a center wavelength, asnoted with LEDs 302 above. Wavelengths within a certain range of thecenter wavelength are typically considered useful, i.e., not prone tocausing errors. Consequently, a useful portion of the spectraldistribution for each LED can be defined as the center wavelength±arange value.

Within a reflectometer, results with respect to reagents are determinedfrom the reflection of light from the source LEDs by the test product.The reflected light, referred to as reflectance, is detected bydetectors, as discussed above with respect to the reflectometer 100 ofFIG. 1 and FIGS. 3A and 3B. The detected reflectance values are afunction of the center wavelengths of the source LEDs. In prior systems,if an LED's center wavelength was outside of the presumed ±3 nm, errorslikely occurred. The likelihood of an error has also been somewhatdependent on the slope of curve of the spectral distribution for thegiven reagent, since some are more immune to variability in the centerwavelength than others. For instance, with a reagent having a fairlyflat slope across a range of center wavelengths that includes the actualcenter wavelength, such as Occult Blood, being off-center would havenegligible effect, typically. But with reagents having relatively highslopes, such as pH and Glucose, an off-center wavelength could producean error condition within the instrument.

Since LEDs are typically purchased in volume, it is impractical tomeasure the range of each LED. For example, a set of one thousand LEDscould be rated or indicated as having a center wavelength of 470 nm.Sampling various LEDs in the set could show that the estimated averagecenter wavelength is 471 nm, not 470 nm. This sampling could also showthat the width of the useful range for this set of LEDs is about 14 nm,not 6 nm, i.e., not the specified range of ±3 nm. Using LEDs 302 aboveas examples, with a set of LEDs corresponding to each LED above, thecenter wavelength and range of the sets of LEDs could be characterizedas follows:

-   -   1) LED 1: 471+6/−7 nm    -   2) LED 2: 525+6/−7 nm    -   3) LED 3: 572+2/−3 nm    -   4) LED 4: 621+3/−3 nm    -   5) LED 5: 652+6/−7 nm    -   6) LED 6: 843+5/−5 nm        In prior systems, LEDs having center wavelengths outside the        specified ±3 nm range would not be used. However, with the        center wavelength correction aspects of the present invention,        adjustments (or “corrections”) can be made to the measured        reflectance values to accommodate center wavelengths, within        ranges of at least about ±8 nm.

For each LED a reference spectral distribution (or LED emissionintensity) is determined. The reference spectral distribution for eachLED is designated as array {L*}, where individual members of {L*} givethe intensity of the light output of the LED at different wavelengths.Therefore, for each of LEDs 302 above, there will be an array {L*}: forLED 1, {L*}={L₄₇₀*}; for LED 2, {L*}={L₅₂₅*}; and so on. Array {L*} isstored in memory for use by a wavelength correction function indetermining the appropriate correction factor to be used in correctingthe actual measured output spectral distribution {L} for each LED. Theactual spectral distribution of light output by each LED used in areflectometer is assumed to be unknown in advanced, and are saved in anarray {L}, comprised of measured L values for each LED. So, as with{L*}, there will be an array for each LED: for LED 1, {L}={L₄₇₀}; forLED 2, {L}={L₅₂₅}; and so on.

FIG. 4 is a graph 400 showing representative plots of LED distributions{L*} for each of the six LEDs, such as LEDs 302 above. In graph 400wavelength in nanometers is represented on the horizontal axis andoutput intensity is represented on the vertical axis. Plot 402corresponds to LED 1, having a center wavelength of about 470 nm. Plot404 corresponds to LED 2, having a center wavelength of about 525 nm.Plot 406 corresponds to LED 3, having a center wavelength of about 565nm. Plot 408 corresponds to LED 4, having a center wavelength of about625 nm. Plot 410 corresponds to LED 5, having a center wavelength ofabout 660 nm. And plot 412 corresponds to LED 6, having a centerwavelength of about 845 nm.

In accordance with aspects of the present invention, an instrument thathas a reference spectral distribution of array {L*} can give rise tocorrected reflectance values R*. But an array of the actual spectraldistribution {L} of the LEDs gives rise to reflectance values R, not R*,since measured reflectance is a function of measured array {L}. That is,without any wavelength correction, R would be the reflectance measured,reported, and used by the instrument in all equations and calculations.Depending on the center wavelength, using R could yield an error, whileusing R* would not. The objective of wavelength correction function is,therefore, to provide a mechanism for determining R* for the instrument,even though the actual LED output distribution in that instrument is{L}, not {L*}, and the measured reflectance values are R. Once R* isdetermined for each LED, R* is used in the functions and equations fordetermining reflectance-based results. These functions and equations areknown in the art, so are not discussed in detail herein.

In accordance with one aspect of the present invention, a correctionfunction c is determined to provide as means of converting R to R*. Theobserved reflectance R and the wavelength corrected reflectance R* areassociated with a single measurement of reflectance at a singlewavelength on a single instrument. Their relationship can be expressedaccording to Equation 1:R*=R·c(R)  (1)alternatively,c(R)=R*/R  (1)

Therefore, R* is the wavelength corrected reflectance, obtained bymultiplying the observed reflectance R by c. And correction functionc(R) can also be expressed as a ratio of corrected reflectance value R*divided by measured reflectance value R, for a given LED.

Equation 1, and the correction function is demonstrated in FIG. 5, whichis a block diagram 500 depicting wavelength correction in accordancewith aspects of the present invention, within the context of thereflectometer of FIG. 1, FIG. 3A and FIG. 3B. As is shown also in FIGS.3A and 3B, the LEDs 302 generate light toward a test product, and thereflected light is received by detectors 360. For each instrument, LEDs302 yield spectral distribution array {L} and the actual, measuredreflectance from the detectors is represented by R. In accordance withaspects of the present invention, the wavelength correction function520, represented as c(R), outputs the corrected reflectance R*, which isthen used by the processor and functions for producing results—given theparticular application of the instrument. The correction function 520determines a correction factor, either from inputs {L*}, {D} and {r} or,alternatively, from a table of predetermiined c(R) values—all of whichis discussed in further detail below. The correction factor is combinedwith the measured reflectance R to determine R*. As will be appreciatedby those skilled in the art, the correction function 520 could beimplemented in software, firmware, hardware, or any combination thereof.

More specifically, Equation 1 can also be expressed as follows:R*=R·c(R/R _(IR) · r _(IR))  (2)where c(R/R_(IR)·r_(IR)) is an alternative way to express the correctionfunction, which does not rely on R*.

R_(IR) is the measured reflectance in the infra red (IR) range andr_(IR) is a constant representing the average IR reflectance. In the IR,the reagents measured typically converge and have a near-zero slope, sothat it becomes possible to assign a constant reflectance value toreflectance readings in the IR. The constant IR reflectance is thusindependent of concentration and wavelength (as long as the wavelengthis within the specified IR range). For purposes herein, from theseconstant IR reflectance values r_(IR) a high resolution referencereagent spectrum {r} is defined, where each value of r in array {r}equals the corresponding value of (R/R_(IR))·r_(IR).

In addition to the high resolution reference reagent spectrum {r}, otherfactors could play a significant role in determining the measuredreflectance R in a reflectometer. Previous modeling has shown that anLED spectral emission {L} and detector sensitivity {D} weighted averageof high resolution reflectance spectra provides an excellent model ofthe measured reflectance R. Accordingly, reflectance R can be calculatedusing the following equation:R=σL _(i) ·r _(i) ·D _(i) /σL _(i) ·D _(i)  (3)where L_(i) are the elements of array {L} for the LED, r_(i) are theelements of an array {r} of high resolution reflectance values for aspecific reagent at a specific concentration, and D_(i) are elements ofthe array {D} of detector sensitivities.

Using the same principles, a similar equation can be written for R*:R*=σL* _(i) ·r _(i) ·D _(i) /σL* _(i) ·D _(i)  (4)where L*_(i) are the elements of array {L*} for the LED referencespectral outputs, which are known. The reference LED spectraldistributions {L*} are generally chosen to have a shape characteristicof the expected distributions of array {L}.

Equation 1 above can be solved for by substituting Equation 3 for R andEquation 4 for R*, as follows: $\begin{matrix}{{c(R)} = \frac{\left( \frac{\sum{L_{i}^{*}r_{i}D_{i}}}{\sum{L_{i}^{*}D_{i}}} \right)}{\left( \frac{\sum{L_{i}r_{i}D_{i}}}{\sum{L_{i}D_{i}}} \right)}} & (5)\end{matrix}$

The values of all variables can be determined. All values of {L*} areknown, as are all values of array {D}. In Equation 5 it is generallyappropriate to use the nominal detector sensitivity curve {D} of thedetectors, which is typically supplied by the detector vendor. Arepresentative plot 600 of {D} is provide in FIG. 6, which provides aplot for CCDs as used in reflectometer 100. In FIG. 6, wavelength isrepresented on the horizontal axis and detector sensitivity isrepresented on the vertical axis.

The high resolution reflectance values of array {r} provide referencereflectance values of the reagent at specific times, for specificanalyte concentrations, and at narrow wavelength intervals, e.g.,typically less than 1 nm. The instrumentally determined reflectancemeasurements R and wavelength corrected reflectance values R* use an LEDas the light source, having typical 20-40 nm bandwidths, which are muchbroader than the interval of the high resolution measurements, so the 1nm increments are sufficiently narrow to provide granularity within thelight source bandwidths.

High resolution reflectance values {r} are generally available at colorblock levels, represented as k. As will be appreciated by those skilledin the art, color block levels are typically predefined for reagent testproducts. For example, if a given test pad on a test strip turns acertain, predefined color, the test may be determined to be positive ornegative, as the case may be. The color indicates a concentration of theanalyte, if present. In the measurement of high resolution reflectance,measurements are made with respect to those concentrations whichcorrespond to the predefined color blocks.

For each wavelength of each color block k of each analyte, R_(k) andc(R_(k)) can be determined. The k designation is used to indicate thedifferent color blocks for the different reagents. For example, thereare 2 blocks for Nitrite and 5 color blocks for Glucose. When themeasured reflectance at a specific wavelength of a specific analyte isR_(k), then the correction factor is c(R_(k)).

The high resolution reflectance values of array {r} at the color blocklevels are generally measured at or near the read times which will beimplemented in the production instrument, e.g., productionreflectometers. Representative plots of {r} at various color blocklevels for Glucose (plot 700) and pH (plot 750) are provided in FIGS. 7Aand 7B, respectively. In FIGS. 7A and 7B wavelength is represented onthe horizontal axis and reflectance (which is ≦1) is represented on thevertical axis. For each color block there is a curve. In plot 700 forGlucose, curve 702 represents r₁ for color block 1, curve 704 representsr₂ for color block 2, curve 706 represents r₃ for color block 3, curve708 represents r₄ for color block 4, and curve 710 represents r₅ forcolor block 5. Table 1 below also shows corresponding data for r₁-r₅.Similarly, in plot 750 for pH, curves 752, 754, 756, 758, 760, 762, 764,and 766 represent r₁-r₈ for color blocks 1-8, respectively.

Options For Determining c(R)

There are at least two options for determining c(R). A first option isto measure the actual distribution {L} of the LEDs used in theinstrument (e.g., reflectometer 100) and perform the calculationsrequired by Equation 5. Since all other variables are known, or can bedetermined from available information, satisfying for {L} is all that isrequired to determine c(R).

A second option is to calculate Equation 5 at fixed LED centerwavelength intervals, and in the production instrument use thosec(R_(k)) values for which the computed center wavelength most closelymatches the measured, or actual, center wavelength. These values can becalculated and entered into a table, such as Table 1 below. However,this option will not suffice in all circumstances, but will work well insome. This is because the second option assumes that the shapes of thespectral distributions of the various LEDs of the production instrumentplay a small role in the correction factors. If c(R_(k)) is not afunction of R, then there will not be a predictable value of c(R) foreach R. This has been observed for pH, where reflectance goes up, thendown with pH.

For example, wavelength sensitivity for Glucose and pH is shown in therepresentative plots 800 and 850 of FIGS. 8A and 8B, respectively. InFIGS. 8A and 8B reflectance is represented on the horizontal axis andc(R) is on the vertical axis. The Glucose plot 800 shows that the c(R)curve changes slowly with R, allowing linear interpolation to be a goodmethod of approximating between known c(R_(k)) values (see Table 1below). However, because of the unique spectra of pH caused by its dualindicators, the plot 850 of the c(R) curve for pH is not a function ofR.

Using this second option, any c(R) curve that is not a fumction of R canbe treated as a special case. For example, pH is a special case. But ithas been identified that only a portion of curve 850 was needed insolving Equation 5. This allows elimination of about half of the points.In doing so, a functional relationship between c and R can be obtained.

Using this process, functional c(R_(k)) tables can be calculated for allcolor block levels of all analytes over a range of about ±8 nm from thecenter wavelength of the reference LED distribution {L*}—here inintervals of 1 nm. This is substantially equivalent to a +1 nm sort ofthe LEDs by center wavelength.

In the preferred form, wavelength correction function c for each LED istabulated every 1 nm in LED wavelengths. Table 1 is a sample for theanalyte Glucose for wavelengths between 468 nm and 479 nm. Here, therange of k is 1 to 5, thus there are reference reflectance values in {r}represented in Table 1 as r1-r5. There is one reflectance value for eachcolor block. Accordingly, for each reflectance value, since c is afunction of R, there is a c(r) corresponding to each r value. TABLE 1Wavelength c(r1) c(r2) c(r3) c(r4) c(r5) r1 r2 r3 r4 r5 468 1.00641.0304 1.0460 1.0389 1.0218 75.1279 48.9656 25.0088 10.2199 5.9157 4691.0042 1.0201 1.0305 1.0261 1.0147 74.9655 48.4772 24.6384 10.0932 5.875470 1.0021 1.0100 1.0151 1.0131 1.0075 74.8069 47.9959 24.2720 9.96555.8329 471 1.0000 1.0000 1.0000 1.0000 1.0000 74.6521 47.5222 23.90989.8369 5.7896 472 0.9980 0.9902 0.9850 0.9869 0.9923 74.5000 47.055123.5517 9.7075 5.7451 473 0.9960 0.9805 0.9702 0.9736 0.9844 74.351246.5956 23.1981 9.5775 5.6993 474 0.9940 0.9710 0.9556 0.9604 0.976374.2055 46.1434 22.8491 9.4471 5.6523 475 0.9921 0.9616 0.9412 0.94710.9680 74.0628 45.6989 22.5049 9.3163 5.6042 476 0.9902 0.9524 0.92700.9338 0.9595 73.9230 45.2619 22.1656 9.1853 5.555 477 0.9884 0.94340.9131 0.9204 0.9508 73.7860 44.8325 21.8314 9.0543 5.5047 478 0.98660.9345 0.8993 0.9071 0.9419 73.6518 44.4109 21.5023 8.9233 5.4534 4790.9848 0.9258 0.8858 0.8938 0.9329 73.5205 43.9971 21.1786 8.7925 5.4011

In practice, tables like that of Table 1 can be provided for allanalytes, and at all wavelengths, in 1 nm increments, for each LED.Although there is a priori knowledge of only the color block reflectancevalues {r} and associated reference reflectance distribution values{L*}, reflectance values between those in the tables must also beaccommodated, such that a corresponding c(R) can be reliably determined.

There are at least three ways for approximating the correction factorc(R) associated with a specific reflectance R using tabulated data, suchas that provided in Table 1.

-   -   1) Round the measured reflectance to the closest tabulated value        and use the associated correction value.    -   2) Interpolate when the measured reflectance value lies between        adjacent points within the table, but use the extreme tabulated        correction factor if the value lies outside the table.    -   3) Interpolate within the table, but extrapolate outside the        table.

For some analytes, there is a significant change in the correctionfactor between points. Therefore, option 1 is typically not preferred.The decision between options 2 and 3 rests on the question of whether itis expected that the correction coefficients level off or continues tochange beyond the ends of the measured reflectance range. For thenegative levels, there, will not be any change because the negative isalready at the extreme of low concentration values. For the positivelevel, most analytes are nearing the maximum change in reflectance.Thus, option 2 is generally preferred.

Regardless of whether c(R) is determined using option 1 or option 2,Equation 1 is used to find R*. Equation 1 provides:R*=R·c(R)  (1)

R is measured by the instrument, as is typically done. c(R) isdetermined as provided above, thus R* is easily calculated. Oncecalculated, R* is the reflectance value used in the typical instrumentcalculations, which ordinarily use R. Accordingly, an error conditionthat would have occurred using R is avoided, and a proper result isachieved with R* determined using the wavelength correction function inaccordance with the present invention.

While the foregoing has described what are considered to be the bestmode and/or other preferred embodiments, it is understood that variousmodifications may be made therein and that the invention or inventionsmay be implemented in various forms and embodiments, and that they maybe applied in numerous applications, only some of which have beendescribed herein. As used herein, the terms “includes” and “including”mean without limitation. It is intended by the following claims to claimany and all modifications and variations that fall within the true scopeof the inventive concepts.

1. A method of correcting one or more reflectance values when a centerwavelength of one or more light sources used to generate correspondinglight signals is different from a specified center wavelength for theone or more light sources, the method comprising the steps of: A.defining, for each of the one or more light sources, a referencespectral distribution {L*} that is characteristic of the one or morelight sources and comprised of reference light intensity values over aset of reference wavelengths; B. defining, for each of the one or morelight sources, a spectral distribution {L} comprising actual lightintensity values over the set of wavelengths; C. determining the actualreflectance R of a set of reflected signals; D. defining a set ofdetector sensitivity data {D} corresponding to the set of detectorsreceiving the set of reflected signals; E. determining high resolutionreflectance values {r}; F. determining a correction factor as a functionof {L}, {L*}, {r} and {D}; and G. applying the correction factor to R todetermine R*.
 2. The method of claim 1, wherein determining thecorrection factor in step F is valid up to a range of at least about ±8nanometers around the specified center wavelength.
 3. The method ofclaim 1, wherein the one or more light sources comprise LEDs.
 4. Themethod of claim 1, wherein at least one of the one or more light sourcesis an infrared light source and determining {r} in step E comprisesmeasuring reflectance values R_(IR) in the infrared range anddetermining r_(IR) as a constant representing an average of R_(IR),where each value in {r} equals the value of (R/R_(IR))·r_(IR) at acorresponding wavelength.
 5. The method of claim 4, wherein the valuesof {r} are determined at discrete wavelength intervals.
 6. The method ofclaim 1, wherein the one or more light sources and set of detectorscomprise part of a reflectometer.
 7. A center wavelength correctionsystem configured to correct one or more reflectance values when acenter wavelength of one or more light sources used to generatecorresponding light signals is different from a specified centerwavelength for the one or more light sources, the system comprising: A.a spectral distribution module configured to determine, for each of theone or more light sources, a spectral distribution {L} comprising actuallight intensity values over the set of wavelengths; B. a reflectancemodule configure to determine actual reflectance R from a set ofreflected signals; C. at least one storage device comprising: 1) foreach of the one or more light sources, a reference spectral distribution{L*} that is characteristic of the one or more light sources andcomprised of reference light intensity values over a set of referencewavelengths; 2) high resolution reflectance values {r}; and 3) detectorsensitivity data {D} corresponding to the set of detectors receiving theset of reflected signals; D. a correction function module configured todetermine a correction factor at a given wavelength as a function of{L}, {L*}, {r} and {D} and to apply the correction factor to R todetermine R*.
 8. The system of claim 7, wherein the correction functionmodule is configured to determine the correction factor within a rangeof at least about ±8 nanometers around the specified center wavelength.9. The system of claim 7, wherein the one or more light sources compriseLEDs.
 10. The system of claim 7, wherein at least one of the one or morelight sources is an infrared light source and the correction function isconfigured to determine {r} as a function of measured reflectance valuesR_(IR) in the infrared range and a constant r_(IR) that represents anaverage of R_(IR), where each value in {r} equals the value of(R/R_(IR))·r_(IR) at a corresponding wavelength.
 11. The system of claim10, wherein the values of {r} are determined at discrete wavelengthintervals.
 12. The system of claim 7, wherein the one or more lightsources and set of detectors comprise part of a reflectometer.
 13. Awavelength correction means for correcting one or more reflectancevalues when a center wavelength of one or more light sources used togenerate corresponding light signals is different from a specifiedcenter wavelength for the one or more light sources, the systemcomprising: A. a spectral distribution means for determining, for eachof the one or more light sources, a spectral distribution {L} comprisingactual light intensity values over the set of wavelengths; B. areflectance means for determining actual reflectance R from a set ofreflected signals; C. at least one storage means for storing: 1) foreach of the one or more light sources, a reference spectral distribution{L*} that is characteristic of the one or more light sources andcomprised of reference light intensity values over a set of referencewavelengths; 2) high resolution reflectance values {r}; and 3) detectorsensitivity data {D} corresponding to the set of detectors receiving theset of reflected signals; D. a correction function means for determininga correction factor at a given wavelength as a function of {L}, {L*},{r} and {D} and to apply the correction factor to R to determine R*. 14.The means of claim 13, wherein the correction function means includesmeans for determining the correction factor within a range of at leastabout ±8 nanometers around the specified center wavelength.
 15. Themeans of claim 13, wherein the one or more light sources comprise LEDs.16. The system of claim 13, wherein at least one of the one or morelight sources is an infrared light source and the correction functionmeans includes means for determining {r} as a function of measuredreflectance values R_(IR) in the infrared range and a constant r_(IR)that represents an average of R_(IR), where each value in {r} equals thevalue of (R/R_(IR))·r_(IR) at a corresponding wavelength.
 17. The systemThe system of claim 16, wherein the correction function means includesmeans for determining values of {r} at discrete wavelength intervals.18. The system of claim 13, wherein wavelength correction meanscomprises a portion of a reflectometer means.
 19. A reflectometercomprising: A. a set of light sources; B. a set of detectors; C. areflectance assembly configured to direct light signals from the set oflight sources onto a test product and to direct light signals reflectedfrom the test product onto the set of detectors; D. at least one storagedevice configured to store a reference spectral distribution {L*}, a setof high resolution reflectance values {r}, a set of detector sensitivitydata {D} corresponding to the set of detectors, a measured spectraldistribution {L} corresponding to the set of light sources, and a set ofmeasured reflectance values R; and E. a correction function module fordetermining a correction factor at a given wavelength as a function of{L}, {L*}, {r} and {D} and to apply the correction factor to R todetermine R*.
 20. The reflectometer of claim 19, wherein the set oflight sources comprises a set of LEDs.
 21. A wavelength correctionmodule, in a reflectance-based system comprising a set of light sources,a set of detectors, and a reflectance assembly configured to directlight signals from the set of light sources onto a test product and todirect light signals reflected from the test product onto the set ofdetectors, the wavelength correction module comprising: A. at least onestorage device configured to store a reference spectral distribution{L*}, a set of high resolution reflectance values {r}, a set of detectorsensitivity data {D} corresponding to the set of detectors, a measuredspectral distribution {L} corresponding to the set of light sources, anda set of measured reflectance values R; and B. a correction functionmodule for determining a correction factor at a given wavelength as afunction of {L}, {L*}, {r} and {D} and to apply the correction factor toR to determine R*.
 22. The wavelength correction module of claim 21,wherein the set of light sources comprises a set of LEDs.